The prior art has taught many configurations of keyboard type switches. A keyboard type switch is inexpensive by nature, due to low current and voltage levels it must switch and minimal emphasis on durability. However, prior art switches each have individual disadvantages together with a common disadvantage.
The basic principle of a keyboard type switch is very simple. There are two electrodes which in their normal or steady state are not connected, that is there is no conductive path between them. In operation, a pushbutton associated with a conductive strip is depressed to urge the conductive strip in position to establish a conductive path between electrodes for a brief period. An additional element, such as a spring, or the configuration of the conductive strip returns the switch to its steady state with the electrodes electrically separated. The rapid contact between electrodes permits an electrical signal to be fed to a microprocessor, solid state memory or the like to key a response. The magnitude of the electrical energy is, in general, very low both in current and voltage.
Individual disadvantages center around the means for returning the conductive strip that bridges the electrodes to its steady state or its normal condition. Some prior art keyboard type switches have used additional springs to urge the conductive strip to its steady state position. However, additional springs can create problems through misalignment. Furthermore, additional springs add cost to the keyboard type switch both in materials and in manufacturing labor man-hours.
The majority of prior art keyboard type switches have eliminated additional springs by configuring the conductive strip to remain in its steady state position and relying on the inherent resiliency of metal to act as a spring to return the conductive strip to its normal condition. Unfortunately, prior art switches which rely upon the resiliency of the conductive strip have a deficiency in the limited spring life of the metal strip and its slow action attributable to a low spring constant.
The limited spring life is due to operating motion being concentrated on a small area of the conductive strip. Moderate use weakens the area of concentration since a metal strip may be bent, even slightly, only a limited number of times. A low spring constant is due also to operation being concentrated on a small area of the conductive strip. The spring constant will be based upon the thickness and the quality of material of the conductive strip. Both factors add cost to the switch to increase the spring constant, which is increased only for a limited time since, as indicated above, the spring weakens with each use.
A general deficiency of prior art switches is that an actuator button is necessary to urge the conductive strip to bridge the electrodes. An actuator button adds material and labor cost to the switch since it is an additional part which is not only added but must be aligned. Furthermore, actuator buttons can accumulate dirt, grease, etc., in depressions and corners associated with a pushbutton. Accumulations of dirt and grease can render keyboard type switches inoperative or, in the alternative, unsuitable for certain uses, such as in the sterile environment of a hospital. Prior Art has attempted to overcome this deficiency by using a single membrane to cover a plurality of switches as described in U.S. Pat. No. 4,086,451 to Henry J. Boulanger. A typical prior art keyboard utilizing a single membrane to cover a plurality of switches is illustrated in FIG. 1.
FIG. 1 depicts a prior art keyboard 2 having a single layered membrane 4 covering a plurality of switches designated by numbered boxes 6. While a single layered membrane may prevent the accumulation of dirt and grease, graphics such as numbers must be placed on either side of the membrane and are subject to wear.
It is therefore an object of the present invention to provide a less expensive keyboard type switch.
It is also an object of the present invention to provide a keyboard type switch which will not accumulate dirt and grease.
It is a further object of the present invention to provide a keyboard type switch requiring no additional urging means to return electrical contacts to their normal steady state position.
It is also a further object of the present invention to provide a simple keyboard switch that overcomes the abovementioned deficiencies of prior art keyboards and keyboard type switches.